Effect of Rayleigh Wave on Ultrasonic Underground Imaging

In order to acquire image with higher resolution and less error for detecting underground objects, a three dimensional ultrasonic underground imaging technique using an electromagnetic-induction type sound source and an amplitude correlation synthesis processing (ACSP) method has been proposed in our previous work. Depending on the conditions of the ground surface, Rayleigh waves propagating along the surface directly may be received with significant amplitude by the receiver array. That will cause interference with the reflect signal of the underground objects and will bring forth error images to the imaging result. In this paper, the Rayleigh wave with comparatively high amplitude is measured experimentally in a model field filled with mountain sands, and its waveform is estimated and simulated approximately by an exponentially decaying sinusoidal wave. The effect on the image of underground object is discussed by synthesizing the received signal with the modeled Rayleigh wave with various relative amplitudes. The result images calculated by ACSP method show that the effect of Rayleigh wave is not marked when its peak amplitude is not greater than that of the signal reflected from the underground object.